Porcellanite is a typical example of a sedimentary rock comprising as a main compound non-crystalline active silica among other constituents considered as impurities. A typical analysis of the main constituents of porcellanite mineral, which exists in Israel is as follows:
SiO.sub.2 : 69% PA1 CO.sub.2 : 7% and PA1 Na.sub.2 O: 0.45%.
Rocks containing non-crystalline silica are found in many places of the world. In Israel, large amounts of pporcellanite are covering the deposits of phosphate rocks near Nahal Zin without being utilized. Moreover, in order to mine the phosphate rock large amounts of porcellanite have to be removed fact which consists a serious ecological problem. Active silica is known as a valuable material useful for many purposes, such as: fillers, extenders, adsorbents, supports, dentrifices, etc.
The literature is quite abundant with many patents describing various approaches for obtaining active silica from alkali metal silicates and a mineral acid such as sulfuric acid or hydrochloric acid. According to the U.S. Pat. No. 3,993,497, precipitated silica is obtained from a sodium silicate solution treated with sulfuric acid and aluminum containing ions.
In the East German Patent Number 293,097, powdered amorphous silica is obtained by reacting alkali metal silicate solutions with an acid or acidic substances, under stirring and adding a mixture of cationic and non-ionic surfactant before reaching an electrolyte concentration of 0.3N.
Although the known processes are yielding various high grades of pure silica, their main disadvantage is the fact that they require a relatively expensive starting reagent such as alkali metal silicate.
It is an object of the present invention to provide a process for obtaining amorphous silica from a natural rock containing it. Another object of the present invention is to provide a proces for obtaining amorphous silica from a natural rock, using unexpensive reagents. It is yet another object of the present invention, to provide a simple process for obtaining amorphous silica of a very high purity from a natural rock containing it. It is a further object of the present invention to provide a simple process for obtaining amorphous silica of a high purity which posssesses a relatively high surface area.